This invention relates generally to high speed digital telecommunications systems, to the creation of virtual rings in a network from existing circuits to provide shared protection for network path segments, to expanding the addressing capability of the SONET path overhead to address more nodes in a virtual ring, to distinguishing between network maintenance alarms from external to a virtual ring versus generated within a virtual ring, and to the use of a multiframe structure to carry more information.
The telecommunications network servicing the United States and much of the world is evolving from analog transmission to digital transmission with ever-increasing bandwidth requirements. Fiber optic cable has proved to be a valuable tool of such evolution, replacing copper cable in nearly every application from large trunks to subscriber distribution plant. Fiber-optic cable is capable of carrying much more information than copper wire, with lower attenuation due to the wide bandpass of fiber optic cable.
It has become increasingly important to maintain communications connectivity in the presence of transmission system failures. To this end, various schemes have been implemented to provide automatic protection for failed network segments by temporarily bypassing such failed network segments.
The disruption of telecommunications services in a telephone network is typically caused by an inoperable communications path (link) or terminal equipment within a service providing office (node). Any disruption of such services can be very costly to business users who rely on telecommunications services in the important operation of their businesses. The duration of a service disruption is typically based on a number of factors, such as, for example, (a) the amount of time required to identify the location of the service disruption; (b) the amount of time that is used to identify one or more routes that could be used to provide at alternate route for affected traffic around the service disruption; and (c) the amount of time that is actually needed to establish such alternate routes. In dealing with a service disruption by selecting an alternate route around an inoperable link or service providing node, one goal is to select the most efficient alternate route, one having least number of nodes and links.
In the prior art a number of service restoration arrangements have been implemented to enhance the reliability of telecommunication networks by providing alternate routes established xe2x80x9con the flyxe2x80x9d to temporarily replace inoperable network links or service providing nodes. Other prior art provide an established backup alternate route to temporarily replace inoperable network links or service providing nodes. In all these service restoration arrangements failed network links or service providing nodes are temporarily bypassed until the failed equipment or cables are repaired.
In today""s self-healing networks, of the types outlined above, four forms of restoration are primarily used: 1) centralized, controller-based, (2) linear protection, (3) path-switched rings (or sub-network connections), and (4) line-switched rings. Because of their speed of restoration, rings tend to be the vehicle of choice. However, rings are not universally deployed for various reasons.
Centralized, controller-based restoration, which takes on the order of thirty seconds to several minutes to restore a faulty circuit, has the advantages of fine granularity and deployment without regard to ring structures. However, these centralized systems are usually coupled to operations systems for failure information, and need to be told when to revert. As the network grows, the controllers become more complex and costly, and are a potential processing bottleneck.
A second type of network protection is a linear arrangement wherein for every primary fiber optic cable in the network that is used to provide service, there is a dedicated, secondary, backup fiber optic cable that provides service in the event there is a fault with the primary cable. A switch over occurs when the signal path in the primary cable, or its terminating equipment, is deemed defective. This is one-hundred percent redundancy which is very expensive. Also, the primary and secondary fibers are typically routed in the same conduit, so a fiber cut likely causes both primary and secondary cables to simultaneously fail. Accordingly, linear protection is not widely used as a network protection scheme for inter-office facilities because of the bandwidth inefficiency and the high probability that both primary and secondary fibers could simultaneously fail.
A third and more commonly used type of network protection utilizes network rings. There are a few types of ring service restoration arrangements. In a first arrangement, called Uni-directional Path Switched Rings (UPSR), telecommunication signals are carried from an originating node to a terminating node through a primary network path comprised of links and intermediate nodes and, at the same time, the signals are carried between the originating and terminating nodes over an alternate ring topology path also comprised of links and intermediate nodes. If there is a failure anywhere in the primary network path or the alternate ring path, there is no loss of signal. This approach is also very expensive and not widely used for inter-office facilities because there is 100 percent redundancy. The primary advantage with this arrangement is there is no need for communications and switching to set up the alternate ring path, because that path is already connected and in use. Therefore, the switch times are on the order of milliseconds. Also, another advantage is that the switching granularity is at the SONET path level, or circuit level. Thus, some of the individual paths within the line can be switched independently based on the health of each signal.
Another type of ring based service restoration, entitled Bi-directional Line Switched Ring (BLSR), utilizes alternate, shared secondary ring bandwidth that is utilized only as needed to provide alternate routes to temporarily replace primary ring traffic due to inoperable network links or service providing nodes. When a fault is detected all primary ring traffic carried on the cable where the fault is detected is switched to the shared secondary alternate ring bandwidth. For example, with reference to FIG. 1, if a failure occurs between nodes E and F, the primary signals within the cable are looped back to node A. The alternate shared secondary bandwidth from node A to node B to Node C to Node D to Node F is utilized to restore the primary ring traffic. The main advantage of BLSR network protection is the shared protection bandwidth. Various primary paths around the ring may utilize the shared protection bandwidth. Therefore, due to the shared protection concept, there are many traffic distribution patterns around the ring that allow much greater bandwidth utilization than path switched rings. The switch times are still on the order of milliseconds, even though the shared protection path needs to be allocated to the appropriate primary signals. A major disadvantage of BLSR is the granularity which is at the SONET line level not circuit level.
Signals carried through fiber-optic cable networks are in accordance with the Synchronous Transport Signal Level (SONET) standard. SONET defines a hierarchy of multiplexing levels and standard protocols which allow efficient use of the wide bandwidth of fiber optic cable, while providing a means to merge lower level DS0 and DS1 signals in a common medium. In essence, SONET establishes a uniform, standardized transmission and signaling scheme which provides a synchronous transmission format that is compatible with all current and anticipated signal hierarchies. The basic SONET signal (STS-1) has a base rate of 51.480 Mb/sec.
The optical equivalent of STS-1 is called Optical Carrier level 1 (OC-1) and is used for transmission across fiber optic cable. The basic STS-1 signal transmission rate can be multiplexed together to form higher signal rates which include STS-3, STS-12, STS-48, STS-192, and beyond. The corresponding optical carrier levels are OC-1, OC-3, OC-12, OC-48, OC-192 and beyond. The STS frame format is composed of 9 rows of 90 columns of 8-bit bytes, for a total of 810 bytes at a 125 microsecond frame rate.
The SONET standard provides a layered operation and its path overhead (administrative) and transport functions are divided into four layers. These layers are the photonic, section, line and path layers. The first three bytes of columns in the SONET STS-1 format are called the transport overhead bytes which include line and section overhead and are used for network maintenance, control and circuit restoration purposes.
The prior art has a BLSR network protection scheme, generally described above, that is at the SONET line layer. Therefore, the switching granularity is at the line layer. Any restoration with this BLSR network protection scheme requires that all paths within the line be restored. Flexibility to switch some of the signals independently from other signals within the line based on the health of the individual signals does not exist. Also, a BLSR network protection scheme must have the same OC-n rate interfaces at all points within the ring in order to function properly.
The BSLR protocol used with SONET utilizes two bytes, a total of sixteen bits, to communicate between nodes in a fiber-optic network. Four of these bits are used to provide a source node identity, four more bits are used to provide a destination node identity, and eight bits are used to carry information between the identified source and destination nodes. With only a four bit address being used for source and destination node addresses, only a maximum of sixteen source and sixteen destination node addresses may be specified. Therefore, there can be no more than sixteen nodes in a Bi-directional Line Switched Ring (BLSR). This is a limitation when ring based circuit restoration is used for a SONET line layer.
The SONET standard also provides for maintenance requirements of the SONET system, including failure detection and reporting using Alarm Indication Signals (AIS). These include failure states of Loss of Signal, Loss of Frame, and Loss of Pointer. There are also other defined AIS signals than those listed in the last sentence. Detection of a failure requires that an AIS signal be generated. The particular AIS signal which is generated depends upon the failure and upon the type of equipment generating the signal. A Line AIS signal is generated by section terminating equipment to alert downstream line terminating equipment that a failure has been detected, while line terminating equipment generates Path AIS. A Path AIS signal is to be generated within 125 microseconds after detection of the failure.
The SONET Line Alarm Indication Signal (L-AIS) indicates to downstream line termination equipment that a signal fail condition of some type has been detected upstream, and further alarms downstream are not required. This is useful in determining where in a line a failure has occurred.
With the SONET standard and the existing ring protocols, it is not possible to create shared protection rings on a portion of a SONET path or a path segment. Creating this type of ring on a path segment basis would allow the most flexibility for network providers to allocate and protect bandwidth. Currently, rings on a path segment basis have the problem of not being able to distinguish failure conditions, such as AIS, caused externally or internally to the path segment based ring. Therefore, the protection ring would provide network protection even though there is no fault in the ring This is one example why path segment based rings are not possible using current approaches.
Thus, in the prior art there is a need for a more efficient ring arrangement to provide protection to service networks without the shortcomings of prior art rings, which are: (a) requiring all interfaces within shared protection based ring to have the same OC-N interface, (b) limiting the granularity of shared protection based rings to the Line Layer, (c) not allowing shared protection based rings to operate on a path segment basis, and (e) the performance bottlenecks of a centralized restoration scheme.
In meeting the above described needs it would be desirable to have restoration based on sub-network connection principles with shared rather than dedicated protection. This protection should, if possible, be granular down to the circuit or path level. Also, the protection should be distributed and autonomous, triggering without information from operating systems and reverting automatically when a fault is cleared. Restoration speed should be in the order of milliseconds. All these are provided with the present invention.
The above described needs in the prior art are satisfied by the present invention. Our invention provides: (a) the ability to have interfaces between different rings that do not have the same OC-N interface, (b) a finer granularity of shared protection based rings to the Path Layer so network providers can allocate bandwidth in a more efficient way, (c) allowing shared protection based rings to be defined on a path segment basis, (d) increasing bandwidth efficiency by using shared protection based rings, and (e) using autonomous, distributed protection rings that automatically and quickly restore service without the need for a centralized controller.
Our invention uses the concept of a virtual ring at the circuit level in a network by designating protection rings independent of general network architecture or the nature of particular network elements. A virtual ring can then be partitioned into sections of working circuits assigned to so-called working path segments of the ring and sections of protection circuits assigned to so-called protection path segments. The protection is enabled via a new virtual ring (VR) embedded signaling protocol at the circuit level. Amongst other improvements, the new VR protocol provides the ability to distinguish between failures incoming to the ring and failures on the path segment based ring. In addition, the new VR protocol is not propagated outside the ring and has no impact on the rest of the network. Thus, the invention can be implemented within a SONET network. Further, a virtual ring utilizing the present invention can be established in a network alongside other prior art rings or within a UPSR. Also, the BLSR protocol adopted for shared, bi-directional protection line switched rings can be modified and applied to the virtual rings. Since the invention allows originating and terminating node address information for the new virtual rings (VR) to be carried in a protocol using a greater number of address bits than used in the prior art, the protocol can be adapted to establish rings having a larger number of ring nodes in each virtual ring.
Virtual rings are distributed across many network elements throughout a network. The interfaces on the network element can be a mixture of any SONET interface rate. The shared protection capability of the virtual ring is utilized when a specific network segment is determined to be defective for some reason. Restoring the primary traffic by rerouting it to the shared protection bandwidth of the virtual ring can be implemented very fast because a central controller is not utilized. This form of shared protection is also more predictable than controller based restoration schemes because it is known with certainty what protection path segments will be preempted to provide network restoration when any particular service path segment is determined to defective. In contrast, network protection schemes utilizing a central controller create are typically slower.
Our novel virtual ring structure can be viewed as a BSLR protection scheme for sub-path entity or path segments within a network. Path segments are a portion of a SONET end-to-end path. Virtual rings can be viewed as a grouping of path segments with a network into a ring structure.
Adapting the BLSR scheme to a path segment level requires two major hardware functions. The first function is an in-band communications channel to transport the new virtual ring (VR) protocol which allows for different network elements within the virtual ring to coordinate the use of the shared protection bandwidth. The second function is the ability to identify a path segment between adjacent nodes. Identification of a path segment is necessary, since only the path segment portion of the path is being protected by the virtual ring. Identifying the path segment also allows for the determination of errors within a virtual ring versus external to the virtual ring and allows for determining the health of the path segment (i.e., number of bit errors on the path segment). Because virtual rings are based on path segments, not the SONET line, both of these hardware functions need to in the Path Overhead (POH), rather than the Line Overhead (LOH), in order that they can pass through intermediate line terminating nodes in the ring to a destination node which may be located several nodes distant.
To provide circuit protection in a network in the most advantageous manner, protection segments of the virtual rings are utilized as needed using existing, low priority, or currently available, service circuit segments to provide backup protection to higher priority path segments in a network. Thus, no dedicated network segments need to be provided to provide automatic protection switching to the network.
When setting up a virtual ring in a network by interconnecting path segments, the number of nodes that are involved in the ring could become quite large. Therefore, a larger number of node addresses are needed than may be addressed using the existing SONET BSLR protocol standard. In accordance with the teaching of the present invention, unused bytes in the Path Overhead (POH) layer of SONET, coupled with multiframe addressing over eight SONET frames, instead of one frame, permit increasing the number of nodes that may be addressed in a ring while still utilizing the BLSR protocol. The number of nodes in a virtual ring can be up to 64 nodes. This is provided by allowing up to 6 bits for a source node identifier and 6 bits for a destination node identifier.
Bits in an unused byte in the POH layer are utilized to: (a) expand the number of nodes that can comprise a virtual ring, (b) indicate the beginning and end of a path segment, (c) carry an Incoming Error Count from the source point of the virtual ring, and (d) provide a virtual ring Automatic Protection Switching (APS) channel for communicating protection switching information between the switching points in the virtual ring.
When a fault occurs on one of the path segments or nodes within our novel virtual ring, APS switching is allowed to occur by detecting that the fault is within the virtual ring and not external to the ring. This is provided by the bits in the POH identifying the source and destination points of the virtual ring. When a fault is external to the virtual ring (VR), automatic protection switching is not allowed to occur within the VR. This increases efficiency of use in the network.